1. Fields of the Invention
The present invention relates to a technology for controlling the temperature of a crystal oscillator using a crystal oscillation device.
2. Description of the Related Art
Conventionally, there is a small crystal oscillator using a constant temperature oven in order to hardly be affected by the change of outside-air temperature. For its realization method, an oven-controlled crystal oscillator (OCXO) for accommodating a crystal oscillator device and its peripheral circuit in a constant temperature oven is known. By adopting the OCXO, an oscillation frequency which is easily changed by the change of outside-air temperature can be electrically corrected (temperature-compensated) to stably oscillate a crystal oscillator.
For example, in order to stabilize the frequency of a general OCXO using an SC-cut oscillation device, a frequency change is set within a temperature range of approximately the maximum temperature in use plus 10° C. (for example, the range C enclosed by a broken line on the temperature characteristic curve (SC-cut curve) shown in FIG. 1). Then, the temperature is compensated around a peak temperature in which the temperature in the set range is most stabilized.
Conventionally, a circuit composing a temperature sensor 101, an amplifier 102, an adder 103, a target temperature setting input terminal 104, an amplifier 105, an amplification adjustment resistor 106, a heater power terminal 107, a heater 108 and a transistor 109 as shown in FIG. 2A, compensates the temperature. Specifically, the amplifier 102 amplifies a voltage value corresponding to a temperature detected by the temperature sensor 101 up to a desired voltage. Then, the adder 103 adds the voltage value and a prescribed voltage inputted from the target temperature setting input terminal 104. Then, the amplifier 105 and the transistor 109 control current that flows through the heater 108 connected to the heater power terminal 107 on the basis of the result to adjust the temperature in the constant temperature oven.
However, in this configuration, when outside air temperature changes, the voltage takes the waveform shown in FIG. 2B (voltage waveform measured at C101). The waveform shown in FIG. 2B indicates a change by a value inputted from the target temperature setting input terminal 104. For example, around the outside-air temperature (ordinarily temperature), the voltage waveform becomes +80° C. and constant within the temperature range C shown in FIG. 1. In this case, the heater 108 controls the temperature of the entire circuit around the crystal oscillator device. If there is no change in the outside-air temperature, the voltage waveform is controlled around the input value from the target temperature setting input terminal 104. However, the outside-air temperature drops or rises, the voltage waveform C101 changes in asymmetry with the input value from the target temperature setting input terminal 104. This occurs due to the change of a thermal response characteristic. For example, if the outside-air temperature exceeds +40° C. when in temperature control, the outside-air temperature is +40° C. and the amount of thermal accumulation and radiation of the entire circuit are the same, the amount of thermal accumulation increases and that of thermal radiation decreases to make the entire circuit difficult to cool and make the temperature stable on the fairly high temperature side.
If conversely the outside-air temperature is less than +40° C., in the temperature control, the amount of thermal accumulation and that of thermal radiation increases to make the entire circuit difficult to warm and make the temperature stable on the fairly low temperature side. Therefore, according to the degree of difference from +40° C. of the outside-air temperature, the degree of the frequency stability changes. In order to prevent the frequency stability from depending on the outside-air temperature as described above, not only an oscillator using a thermally conductive plate in order to conduct the heat of a heat source and a crystal oscillation device, shown in FIG. 3A but also an oscillator with a constant temperature oven, so-called single-oven-structured oscillator, shown in FIG. 3B, are proposed.
In order to improve the stability against the change of the outside-air temperature, the correction value of each temperature measured by the thermistor or the like and is stored in memory, which is not shown in FIG. 3. Alternatively, the voltage of a variable-capacity diode for converting the value from digital to analog according to the outside-air temperature and controlling the crystal oscillation device in the oscillator can be controlled.
FIGS. 3A and 3B are described below. FIG. 3A shows the partial cross-section view of an oscillator using a thermally conductive plate and the flat view of its major part. FIG. 3B is the partial cross-section view of a single-oven-structured oscillator. The circuit substrate 601 of the oscillator shown in FIG. 3A is covered with a metal base 602 and a metal cover 603. The circuit substrate 601 comprises a crystal oscillation device 604 and its peripheral circuit. The peripheral circuit comprises an oscillation circuit, a temperature compensation circuit and a heating source. The oscillation circuit oscillates the crystal oscillation device 604. The thermal compensation circuit suppresses the influence on the circuit of a temperature change and controls the crystal oscillation device to stably oscillate. For the thermally conductive plate 608, a metal plate, such as an aluminum plate or the like is used. For example, as shown in FIG. 3A, a hollow is provided at each of one set of ends of one thermally conductive plate 608 and at each of the other set of ends orthogonal to this. An opening 605 is also provided in the center area. A through hole 607 through which the lead wire 606 of the crystal oscillation device 604 is provided in the outer circumference of the opening 605. Then, the four corners of the thermally conductive plate 608 are fixed on the circuit substrate 601 by screws, which are not shown in FIG. 3A. In this example, thermally conductive resin is spread between the circuit substrate 601 and the thermally conductive plate 608, which is not shown in FIG. 3A to thermally combine them.
In FIG. 3A, two heating sources (chip resistors 609a and 609b) are provided and is used a heating resistor by joule heat. They are disposed at one set of hollows of the thermally conductive plate 608 mounted on the circuit substrate 601. A transistor (power transistor) 610 used to heat is disposed at each of the other set of hollows of the thermally conductive plate 608. The chip resistors 609a and 609b and the transistor 610 are covered with thermally conductive resin. In this case, resin is also coated from the thermally conductive plate 608, and the chip resistors 609a and 609b, the transistor 610 and the thermally conductive plate 602 are thermally combined.
The respective thermally high-sensitive devices (variable-voltage capacitor device and thermistor) of the transistor 610 and a temperature detection device 611 are disposed at the opening 605 provided in the center area of the thermally conductive plate 608 and are thermally combined with the thermally conductive plate 608. The transistor 610 excluding the thermally high-sensitive devices is disposed at the other main surface of the circuit substrate 601, opposed to the thermally conductive plate 608, and the temperature detection device 611 is disposed in the outer circumference of both the surfaces of the circuit substrate 601.
FIG. 3B shows a single-oven-structure oscillator. The oscillator is provided for a substrate 612, and by covering the entire substrate with a metal base 613 and a metal cover 614 and by maintaining the inside temperature constant by controlling the temperature of a heater, such as a heating wire 615 or the like, the frequency is stabilized.
Patent reference 1 proposes a method for simplifying the component management by widely reducing the power consumption of the oscillator using a constant temperature oven and selecting the temperature detecting place of a thermally sensitive device for detecting the temperature inside/outside the constant temperature oven (inside/outside the constant temperature oven) depending on the specification of the thermally sensitive device.
According to patent reference 2, a piezo-electric vibrator, an amplification circuit and an oscillation circuit using a variable-capacity diode are disposed in a constant temperature oven in order to maintain the temperature of the piezo-electric vibrator constant. Then, a voltage generation circuit for outputting a control voltage in order to control the capacity value of a variable-capacity diode in such a way as to suppress the fluctuations of the oscillation frequency of the piezo-electric oscillator due to the change of the electrical characteristic of the amplification circuit accompanying a temperature change is further provided. The voltage generation circuit uses a thermistor with a plus or minus characteristic as a thermally sensitive device and compensates for a frequency change accompanying the change of the electrical characteristic of the amplification circuit due to the temperature change by controlling a control voltage. Thus, patent reference 2 proposes a method for easily realizing a small constant temperature oven type crystal oscillator which is superior in a frequency-temperature characteristic as described above.
Patent reference 3 proposes a method for obtaining a stable frequency, reducing the heat conduction from the outside, maintaining the temperature of the constant temperature oven stable and obtaining a stable frequency by reducing the loss of heat by reducing the number of the supporting point of a constant temperature oven to one and obtaining a more stable frequency by attaching a sensor to a support metal and anticipating a future temperature change from the temperature difference from the outside and controlling the temperature of the constant temperature oven.
However, in the case of a single-oven-structured oscillator, the proposals described in patent references 1-3 have a problem that the temperature in a constant temperature oven (of the entire circuit) is easy to change against the change of the outside-air temperature.
In order to shut the change of the outside-air temperature outside the oven for maintaining the temperature of the entire circuit around the crystal oscillation device and to maintain high stability even when the outside-air temperature changes, the single-oven structure shown in FIG. 3B or a double-oven structure can be adopted as another method. However, either of them has a disadvantage that power consumption increases.
Patent reference 1: Japanese Patent Application No. H10-303645
Patent reference 2: Japanese Patent Application No. 2002-135051
Patent reference 3: Japanese Patent Application No. 2005-159797